Eleven hours a day, seven days a week, Cynthia McKay maintains a clockwork schedule. From 7 a.m. to 6 p.m., she is in her office as CEO of LeGourmet Gift Basket in Castle Rock, Colo. That adds up to a 77-hour workweek, not counting her time at home on 24-hour call for clients around the world.

But don't feel sorry for Ms. McKay. Her long hours are "absolutely my choice," she says, adding, "I love being at work. It becomes a lifestyle as opposed to a job."

That kind of schedule puts McKay in the rarefied company of a growing group of highly paid professionals who hold "extreme" jobs. One-fifth of high earners surveyed in the United States have such jobs, according to new research. In addition to logging 60 or more hours a week, many travel regularly, maintain fast-paced, unpredictable schedules, and respond to clients' demands around the clock.

Although workaholics have always existed, their image has been glamorized. Today's overachievers are cast as "road warriors and masters of the universe," says Sylvia Ann Hewlett, president of the Center for Work-Life Policy in New York. Yet despite their impressive financial rewards, those burning this midnight oil face challenges. Warning that their pace is not sustainable, Dr. Hewlett says, "There's a lot of risk attached. The fallout in private lives is huge." In addition, women are being left behind because many cannot put in 70-hour weeks.

Hewlett's report, "Extreme Jobs: The Dangerous Allure of the 70-Hour Workweek," is published today in the Harvard Business Review.

What distinguishes these overachievers is their passion for their work. Two-thirds of high earners in a range of professions in the US and three-quarters of top managers in multinational corporations say they love their jobs.

"The big surprise of the data was just how much these extreme professionals love their work," Hewlett says. "It is a knowledge economy. Millions of people are amazingly challenged and stimulated by their work. That is good news."

Extreme jobs exist everywhere – in large manufacturing companies and small firms; in law, medicine, entertainment, media, technology; and on Wall Street.

Beyond extreme workers' personal ambitions, several cultural factors are helping to drive the trend, Hewlett says. One is globalization, which requires professionals to work across multiple time zones. Communication technology also plays a role, allowing workers to stay in constant contact. Increased competition for high-level positions and declining job security also encourage excessive work.

Extreme jobs do not exist in a vacuum. "We're not just in an age of extreme work, we're in an age of extreme culture," says Catherine Orenstein, a cultural critic in New York. She points to the popularity of extreme sports, extreme parenting, and extreme reality shows.

In the beginning, Ms. Orenstein says, extreme sports offered "high risk for the fast and the few." Now they also appeal to those with ordinary skills. "We have reality TV shows like 'Fear Factor,' where average people do death-defying stunts. Even at the gym you can see sort of a trickle-down of this."

Today the extreme metaphor has spread to work and popular ideas of success, Orenstein says. " 'American Idol,' 'Project Runway,' and 'The Apprentice' are all shows about how you make it in America. Anyone with hard work, determination, risk, individualism, and bravado will be rewarded. But there's only one winner, so it's a jackpot idea."

In the 1950s, the popular cultural ideal of work and success was far different. "We had a notion of moderation," Orenstein says. "You can see it in 'Ozzie and Harriet.' For a reasonable amount of work over a reasonable amount of time, you have a reasonable amount of security and a reasonable amount of success. Now we have a more extreme notion of success and the amount of work that should go into it."

But these nonstop schedules require sacrifices that intrude on personal and family life. More than one-third of high-earning individuals work more than 60 hours a week, and most report that they are putting in an average of 16.5 hours per week more now than they did five years ago.

"There's a big undermining of personal health, whether it's addiction to sleep medications or crazy diets because you're traveling all over the world," Hewlett says. More than two-thirds of professionals surveyed do not get enough sleep. A significant number overeat.

Michael Bernstein, a physician in New York, sometimes works 80 hours a week. During those periods, he says, "There's no time for cooking and preparing food in a healthy way. We eat out or order in."

Extreme jobs can hurt relationships, Hewlett finds. Laura Stack, author of "Leave the Office Earlier," notes that the American Academy of Matrimonial Lawyers identifies a preoccupation with work as one of the top four causes of divorce.

McKay appreciates the flexibility of her husband, Paul Gomez, who is the assistant attorney general for Colorado. Calling him "a saint who tolerates the schedule well," she notes that they talk often during the day and spend time together in the evening.

For those with extreme jobs like hers, other activities must go. "I used to play golf, but I've given that up to do the work," she says. "We never go out, so we splurged and built a movie theater in our house." And vacations? Not a chance.

McKay has plenty of company. Among extreme job holders, 42 percent take 10 or fewer vacation days a year. More than half say they regularly cancel vacations.

Although Susan Cunningham works long hours as a vice president at an interior architecture firm in New York, she takes vacations. "It is imperative to find time outside of work to de-stress," she says.

Alexander Southwell, a federal prosecutor for the US Attorney's Office in New York, typically logs 70-hour weeks. "When I'm on trial, it can be 80, 90, 100 hours a week," he says. "It can be 8 a.m. to midnight every day, often seven days a week."

It is a pace he has kept up for about five years, the typical length of time people stay in his office. "It's very intense. It's very hard to keep up with things in your personal life. I don't go to the gym. I often skip meals. It's hard. It's very draining."

But the professional tug remains strong. "Part of the reason I stay and part of the reason I do it is it's an amazing job. I feel very passionate about it," says Mr. Southwell.

He and his wife have two young children. "I try to get them dressed and fed in the morning to let my wife relax," he says. "It's important to spend that time with them. Sometimes I will take my older child to nursery school."

For some children, parents' all- consuming careers pose problems. "Kids are very conscious that many of their parents are stressed and tired," says Ellen Galinsky, president of the Families and Work Institute in New York, who interviewed children of working parents for her book "Ask the Children."

When family problems do arise, women are much more likely to notice the problems than men are.

"Both men and women see that children are underperforming at school, eating junk food, and watching too much TV," Hewlett says. "Men don't take this personally. But maternal guilt is alive and well. Women draw a straight line between problems in their own lives and their jobs." That makes them more likely to leave their careers. Fifty-seven percent of women holding extreme jobs do not want to continue the pace for more than a year.

"A business model that requires top people to put in 70-hour workweeks for decades at a time seriously excludes women," Hewlett says. "It kind of explains why progress has stalled for women."

Noting the need to rethink careers, Ms. Galinsky says, "We need to create more flexibility in careers so you can step sideways or go full steam ahead for a while."

Hewlett describes a program at American Express called Project Embrace. "An individual who needs a breather or needs to step back from one of these intense jobs for two years can do a project that is not full time."

Ms. Stack argues for attitudinal shifts as well. "We overindulge in work to the exclusion of life. What will happen to these people if they get a pink slip? They have gotten to the point that they don't know what life is, because work is their life.

"Overwork has become a competition. I don't think it's a competition worth winning."